Our ultimate goal is to understand the molecular basis of a variety of genetic phenomena in Drosophila that result in quantitative changes of certain sequences, such as the ribosomal RNA genes, in the genome of either germ line or somatic cells. A specific set of genetic conditions are required to drive these increases or decreases in abundance. However, what appears to be central to all of them is aberrant chromosome pairing mediated by the presumptive pairing site called cr ion. By a variety of experiments, we intend to clarify the role of cr ion in such genetic phenomena as gene compensation, gene magnification, the abo effect and position effect variegation. Further, we intend to isolate by cloning techniques the pairing sites associated with intercalary heterochromatin and to characterize these sequences in detail. With these cloned pairing sites, we intend to construct an in vitro chromosome pairing system. By understanding how specific sequences recognize each other, it may be possible to gain insight into the general mechanism by which chromosomes pair with one another as well as how cr ion functions.